Variable gain control transformer and RF transmitter utilizing same

ABSTRACT

According to one embodiment, a variable gain control transformer comprises a primary winding connected to differential inputs of the variable gain control transformer, a secondary winding for providing a single ended output to a load, and an output control circuit coupled to the secondary winding, the output control circuit configured to provide up to approximately 12 dB of gain control. Variable gain control may be achieved using first and second variable resistors of the output control circuit, wherein the first and second variable resistors are implemented by respective first and second pluralities of source-drain resistances produced by respective corresponding first and second pluralities of selectable field-effect transistors (FETs). In one embodiment, the variable gain control transformer further comprises a variable capacitance tuning circuit coupled between the differential inputs, the variable capacitance tuning circuit implemented using a plurality of selectable fixed capacitance unit cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of electronic circuitsand systems. More specifically, the present invention is in the field ofcommunications circuits and systems.

2. Background Art

Transceivers are typically used in communications systems to supporttransmission and reception of communications signals through a commonantenna, for example at radio frequency (RF) in a cellular telephone orother mobile communication device. A transmitter routinely implementedin such a transceiver in the conventional art may utilize severalprocessing stages to condition and preamplify a transmit signal prior topassing the transmit signal to a power amplifier (PA). For example, thetransmit signal may originate as a digital signal generated by a digitalblock of the transmitter. That digital signal is then typicallyconverted into an analog baseband signal, by means of adigital-to-analog converter (DAC) for instance. The analog basebandsignal may then be filtered using a low-pass filter (LPF) andup-converted to RF by a mixer, which is usually implemented as an activecircuit. Subsequently, the up-converted signal can be processed by a PAdriver, which then passes the preamplified transmit signal to the PA forfinal amplification and transmission from the transceiver antenna

In a conventional transmitter preamplification chain, pre-amplificationgain control may be approximately evenly distributed between lowerfrequency gain control stages implemented prior to, or in combinationwith up-conversion, and higher frequency gain control stages afterup-conversion. In that conventional design approach, the DAC, LPF, andmixer circuits may collectively contribute a significant portion of theoverall gain control, such as approximately fifty percent of thepreamplification gain control, for example. Unfortunately, thisconventional approach to providing preamplification gain control isfraught with significant disadvantages, owing in part to the substantialinefficiencies resulting from the time and iterative testing required tocoordinate calibration amongst the various lower frequency and higherfrequency gain control stages.

Despite the disadvantages associated with conventional approaches, thoseapproaches remain in widespread use due to the challenges posed byachieving adequate transmitter pre-amplification gain control at higherfrequency using the PA driver stages alone. In order to overcome thosechallenges, it is desirable to obtain some gain control contributionfrom each stage of the PA driver, including its output stage to the PA.Thus, there is a need to overcome the drawbacks and deficiencies in theart by providing a variable gain control transformer for implementationas an output transformer of an RF transmitter PA driver and suitable foruse in a more modern mobile communication device.

SUMMARY OF THE INVENTION

The present invention is directed to a variable gain control transformerand RF transmitter utilizing same, substantially as shown in and/ordescribed in connection with at least one of the figures, and as setforth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmitter including a power amplifier(PA) driver utilizing a variable gain control transformer, according toone embodiment of the present invention.

FIG. 2 is a block diagram of a variable gain control transformersuitable for implementation as an output stage of a PA driver, accordingto one embodiment of the present invention.

FIG. 3 is a block diagram of a variable capacitance tuning circuitsuitable for implementation as part of a variable gain controltransformer, according to one embodiment of the present invention.

FIG. 4A is a diagram showing elements of an output control circuitsuitable for implementation as part of a variable gain controltransformer, according to one embodiment of the present invention.

FIG. 4B is a diagram showing a sub-circuit for producing a variableresistance used in the output control circuit of FIG. 4A, according toone embodiment of the present invention.

FIG. 5 is a table showing control signals used in conjunction with theoutput control circuit of FIG. 4A for producing variable gain control bythe transformer, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a variable gain control transformerand RF transmitter utilizing same. Although the invention is describedwith respect to specific embodiments, the principles of the invention,as defined by the claims appended herein, can obviously be appliedbeyond the specifically described embodiments of the invention describedherein. Moreover, in the description of the present invention, certaindetails have been omitted in order to not obscure the inventive aspectsof the invention. The details left out are within the knowledge of aperson of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. To maintain brevity, other embodiments of the invention,which use the principles of the present invention are not specificallydescribed in the present application and are not specificallyillustrated by the present drawings.

FIG. 1 shows a block diagram of transmitter 100 including a poweramplifier (PA) driver implementing a variable gain control transformer,according to one embodiment of the present invention capable ofovercoming the disadvantages associated with conventional designs. Asmay be seen from FIG. 1, transmitter 100 may be configured to supportmultiple transmission modes and/or multiple transmission frequencies.For example, transmitter 100 can be configured to support high-bandtransmission frequencies in a range between approximately 1.8 GHz and2.2 GHz, as well as low-band transmission frequencies ranging betweenapproximately 0.8 GHz and 1.1 GHz. It is noted that the implementationalarrangement shown in FIG. 1, as well as the circuits represented byFIGS. 2, 3, 4A, and 4B of the present application, are for the purposeof assisting in the understanding of and conveying various concepts ofthe present invention. Elements shown in those figures arerepresentations of physical and electrical elements used in implementingvarious embodiments of the present invention.

As shown in FIG. 1, transmitter 100 includes PA 196, which can becoupled to an antenna utilized by transmitter 100 (antenna not shown inFIG. 1). As further shown in FIG. 1, transmitter 100 includes afront-end comprising digital block 102 providing in-phase (I) andquadrature phase (Q) output signals, such as baseband signals, forexample, to respective digital-to-analog converters (DACs) 104 a and 104b. In addition, and as also shown in FIG. 1, transmitter 100 includesadjustable low-pass filters (adjustable LPFs) 108 a and 108 b. Tosupport high-band frequency channels as well as low-band frequencychannels, transmitter 100 includes high-band mixer 110 a and low-bandmixer 110 b, which may be implemented as passive circuits, for example.In addition, transmitter 100 includes high-band variable gain control PAdriver 120 a and low-band variable gain control PA driver 120 b.

As further shown in FIG. 1, high-band variable gain control PA driver120 a and low-band variable gain control PA driver 120 b compriserespective variable gain control transformers 130 a and 130 b includingrespective variable capacitance tuning circuits 170 a and 170 b. Each ofvariable gain control transformers 130 a and 130 b includes a primarywinding for receiving differential input signals from an amplificationcircuit of its respective PA driver, as well as a secondary winding toprovide a preamplified transmit signal as a single-ended output to PA196.

Also shown in FIG. 1 are transmitter phase-locked loop (TX PLL) 106 andlocal oscillator generator (LOGEN) 107, as well as differential feedbackcalibration stage 190 providing calibration feedback data 192 a and 192b to digital block 102 through analog-to-digital converter (ADC) 194. Itis noted that although TX PLL 106 and LOGEN 107 are shown in duplicatein FIG. 1 for the purposes illustrative clarity, in practice, a singlecombination of TX PLL 106 and LOGEN 107 can be coupled to bothrespective high-band and low-band variable gain control PA drivers 120 aand 120 b, and can be shared by respective high-band and low-band mixers110 a and 110 b as well. It is further noted that although theembodiment of FIG. 1 characterizes baseband signal generation as beingto performed digitally by digital block 102, that need not be the casein all embodiments. Thus, digital block 102 may be seen to correspondmore generally to any suitable baseband signal generator.

According to the embodiment of FIG. 1, substantially all of thepreamplification gain control required for transmitter 100, such asapproximately 80 dB, or more, of preamplification gain control, can beprovided by each of variable gain control PA drivers 120 a and 120 b athigh frequency, e.g., after up-conversion by respective high-band andlow-band mixers 110 a and 110 b. Moreover, variable gain controltransformers 130 a and 130 b can be configured to provide up toapproximately 12 dB of the preamplification gain control provided bytheir respective PA drivers. In addition, each of variable gain controltransformers 130 a and 130 b can be further configured to substantiallymatch the input impedance of its load. For example PA 196, which loadsvariable gain control transformers 130 a and 130 b, may present an inputimpedance of approximately fifty ohms (50Ω), and variable gain controltransformers 130 a and 130 b can be configured to substantially matchthat input impedance value while providing up to approximately 12 dB ofgain control.

As mentioned above, the embodiment of FIG. 1 may be implemented tosupport multiple transmission modes, such as transmission modesemploying quadrature modulation schemes and transmission modes employingpolar modulation, for example. For instance, in FIG. 1, transmissionmodes employing quadrature modulation can be associated with the solidline signal paths linking I and Q outputs of digital block 102 tovariable gain control PA drivers 120 a and 120 b through respectiveDAC/adjustable LPF/mixer combinations 104 ab/108 ab/110 a and 104 ab/108ab/110 b. Analogously, transmission modes employing polar modulation canbe associated with the dashed line signal paths linking digital block102 to variable gain control PA drivers 120 a and 120 b through TX PLL106. In one embodiment, at least one of the multiple transmission modesoperable using transmitter 100 supports both data-band and voice-bandcommunications.

It is noted that although some of the pre-PA signal paths shown in FIG.1 are represented by single lines for simplicity, many of those signalscan comprise paired differential signals. Thus, the I and Q outputs ofdigital block 102 passed to respective high-band and low-band mixers 110a and 110 b, the outputs of respective high-band and low-band mixers 110a and 110 b, the polar mode outputs of digital block 102 passed tovariable gain control PA drivers 120 a and 120 b through TX PLL 106, andthe feedback calibration signal returned to digital block 102 by ADC194, for example, can comprise differential signals. Thus, variable gaincontrol PA driver 120 a and 120 b may be differentially driven. It isfurther noted that the signal paths internal to variable gain control PAdrivers 120 a and 120 b, as well as the feedback signals provided bythose variable gain control PA drivers to differential feedbackcalibration stage 190 and the calibration feedback data 192 a and 192 boutput by feedback calibration stage 190 are explicitly shown asdifferential signals.

As further shown in FIG. 1, the I and Q signal paths provided byrespective DACs 104 a and 104 b and adjustable LPFs 108 a and 108 b canbe shared between the high-band and low-band transmission signals. Inaddition, a single implementation of each of digital block 102, TX PLL106, LOGEN 107, feedback calibration stage 190, ADC 194, and PA 196 maybe used to support all transmission modes and all transmission frequencybands. Moreover, variable gain control transformers 130 a and 130 b maybe used to support all transmission modes for the respective high-bandand low-band transmission frequency bands. Consequently, transmitter 100is characterized by a compact space saving architecture that may beparticularly well suited to meet increasingly fine dimensional and lowerpower consumption constraints as fabrication technologies transition tothe 40 nm node, for example, and beyond.

Transmitter 100 may be implemented as part of a communicationstransceiver, for example, utilized in a cellular telephone or othermobile communication device operating at RF, such as in a frequencyrange from approximately 0.8 GHz to approximately 2.2 GHz. Moreover, inone embodiment, transmitter 100 can be implemented as part of atransceiver integrated circuit (IC) fabricated on a single semiconductorwafer or die using a 40 nm process technology, for example.

Turning now to FIGS. 2, 3, 4A, and 4B, those figures show an exemplaryvariable gain control transformer and variable gain control transformerconstituent circuits in greater detail, according to embodiments of thepresent invention. For example, FIG. 2 is a representation of a variablegain control transformer and FIG. 3 shows an example of a variablecapacitance tuning circuit suitable for implementation in the variablegain control transformer of FIG. 2, according to embodiments of thepresent invention. Moreover, FIG. 4A shows an output control circuitimplemented as part of a variable gain control transformer, while FIG.4B shows a high-precision resistor implemented using the source-drainresistance presented by the controlled operation of a field-effecttransistor (FET), according to embodiments of the present invention.

Referring first to FIG. 2, FIG. 2 shows a block diagram of elements ofvariable gain control transformer 230, which can be seen to correspondto either or both of variable gain control transformers 130 a and 130 b,in FIG. 1. Variable gain control transformer 230, in FIG. 2, includes avariable capacitance tuning circuit shown as cap array 270, whichcorresponds to either or both of variable capacitance tuning circuits170 a and 170 b, in FIG. 1. As shown in FIG. 2, variable gain controltransformer 230 also includes primary winding 232 connected todifferential inputs 222 and 224 of variable gain control transformer230, and receiving differential inputs from an amplification circuit ofa PA driver, such as a transconductance amplifier or current steeringblock of one of PA drivers 120 a and 120 b in FIG. 1 (amplificationcircuit and PA driver not explicitly shown in FIG. 2).

As further shown in FIG. 2, variable gain control transformer 230 alsoincludes secondary winding 240 and output control circuit 241 includingvariable resistors 246 and 248 coupled to secondary winding 240. Alsoshown in FIG. 2 are outputs 242 and 244 for selectably providing asingle-ended output to a PA, such as PA 196, in FIG. 1 as V_(OUT) (PAalso not explicitly shown in FIG. 2), wherein one or the other ofoutputs 242 and 244 are selected according to the transmission modebeing used by transmitter 100, in FIG. 1.

For example, when used to support a first transmission mode, such asWideband Code Division Multiple Access (W-CDMA), a single-ended outputmay be taken from output 242 of variable gain transformer 230, whileoutput 244 is grounded. Alternatively, when used to support anothertransmission mode, such as Global System for Mobile communications (GSM)or Enhanced data rates for GSM Evolution (EDGE), for example, asingle-ended output may be taken from output 244 of variable gaintransformer 230, while output 242 is grounded. Thus, variable gaincontrol transformer 230 can be configured to selectably support multipletransmission modes providing voice-band and data-band communicationswhen implemented as part of a PA driver in an RF transmitter, e.g.,transmitter 100, in FIG. 1.

Gain control for variable gain control transformer 230, in FIG. 2, maybe provided by output control circuit 241, which may be configured toprovide up to approximately 12 dB of gain control, for example. Outputcontrol circuit 241 can provide variable gain control through use ofvariable resistors 246 and 248, as will be further described below inconjunction with FIGS. 4A and 4B. Moreover, in order to support themultiple transmission modes of transmitter 100, in FIG. 1, as describedabove, it is desirable that variable gain transformer 230 be tunable.That tuning function may be provided by cap array 270 coupled betweendifferential inputs 222 and 224 of variable gain control transformer 230and configured to operate as a variable capacitance tuning circuit forvariable gain control transformer 230. The operation of cap array 270 isdescribed in greater detail in conjunction with FIG. 3.

Referring to FIG. 3, FIG. 3 is a block diagram showing variablecapacitance tuning circuit 370 suitable for implementation as part of avariable gain control transformer, such as variable gain controltransformers 130 a and 130 b in FIG. 1, or variable gain controltransformer 230 in FIG. 2, according to one embodiment of the presentinvention. Variable capacitance tuning circuit 370 corresponds to eitheror both of variable capacitance tuning circuits 170 a and 170 b, in FIG.1, as well as to cap array 270, in FIG. 2.

According to the embodiment shown in FIG. 3, variable capacitance tuningcircuit 370 comprises “n” selectable fixed capacitance unit cells 380,each coupled between differential inputs to the transformer primary,e.g., between differential inputs 222 and 224 of variable gain controltransformer 230, in FIG. 2. Each selectable fixed capacitance unit cell380, in FIG. 3, includes first and second fixed unit capacitors 382 aand 382 b, each having a corresponding plate selectably coupled toground by respective first and second switches 384 a and 384 b. Asfurther shown in FIG. 3, for each selectable fixed capacitance unit cell380, node 383 a is shared by fixed unit capacitor 382 a and switch 384a, and node 383 b is shared by fixed unit capacitor 382 b and switch 384b. Moreover, nodes 383 a and 383 b can be selectively coupled to asupply voltage, e.g., V_(DD) by a common third switch 388 throughrespective resistors 386 a and 386 b which may be approximately fortykilo-ohm (40 kΩ) resistors, for example. That is to say, a firstselectable fixed capacitance unit cell 380 comprises fixed unitcapacitors 382 a(1) and 382 b(1), nodes 383 a(1) and 383 b(1), resistors386 a(1) and 386 b(1), and switches 384 a(1), 384 b(1), and 388(1), andso forth, until finally the nth selectable fixed capacitance unit cellcomprises fixed unit capacitors 382 a(n) and 382 b(n), nodes 383 a(n)and 383 b(n), resistors 386 a(n) and 386 b(n), and switches 384 a(n),384 b(n), and 388(n).

Focusing on the first selectable fixed capacitance unit cell 380comprising fixed unit capacitors 382 a(1) and 382 b(1), nodes 383 a(1)and 383 b(1), resistors 386 a(1) and 386 b(1), and switches 384 a(1),384 b(1), and 388(1) as representative of the operation of each of the nselectable fixed capacitance unit cells comprised by variablecapacitance tuning circuit 370, it is first noted that each of theswitching devices is represented as an n-channel FET (NFET), such as ametal-insulator-semiconductor FET (MISFET) or N typemetal-oxide-semiconductor (NMOS) switching device. That representationis merely exemplary, however, and other embodiments of variablecapacitance tuning circuit 370 can be implemented using other types ofswitching devices.

Referring to the specific configuration shown in FIG. 3 to illustratethe operation of selectable fixed capacitance unit cell 380, accordingthat embodiment, when switches 384 a(1) and 384 b(1), e.g., NMOSswitches, receive a HIGH control signal at their respective gates,switch 388(1) is LOW and selectable fixed capacitance unit cell 380 isswitched in. In that case the direct-current (DC) voltage of nodes 383a(1) and 383 b(1) is at ground, and the alternating-current (AC) voltageat those nodes is almost zero. However, when switches 384 a(1) and 384b(1) are controlled LOW, switch 388(1) is sent HIGH, and selectablefixed capacitance unit cell 380 is switched out. That is the casebecause when switches 384 a(1) and 384 b(1) are LOW while switch 388(1)is HIGH, fixed unit capacitors 382 a(1) and 382 b(1) are each in serieswith respective resistors 386 a(1) and 386 b(1), which may be verylarge. Consequently, at RF selectable fixed capacitance unit cell 380presents a substantially open circuit when switches 384 a(1) and 384b(1) are LOW while switch 388(1) is HIGH. Moreover, the DC voltage atnodes 383 a(1) and 383 b(1) is set to VDD, e.g. approximately 1.2V,which ensures that the junction diodes of MOS switches 384 a(1) and 384b(1) remain reverse-biased.

Variable capacitance tuning circuit 370 may comprise any suitablenumber, n, of selectable fixed capacitance unit cells 380, such as fromapproximately twenty to approximately thirty [(˜20)-(˜30)] selectablefixed capacitance unit cells, for example. Implementation of variablecapacitance tuning circuit 370 in a variable gain control transformer,such as variable gain control transformer 230, in FIG. 1, enables asingle variable gain control transformer used for high-band or low-bandtransmission frequency operation to selectably support several or all ofthe multiple transmission modes used by a transmitter. Referring to FIG.1 and the example embodiment in which transmitter 100 is configured tooperate in any of W-CDMA, GSM, and EDGE transmission modes, for example,implementation of the architecture shown in FIG. 3 as variablecapacitance tuning circuits 170 a and 170 b advantageously enables eachof respective variable gain control transformers 130 a and 130 b tosupport all of the transmission modes used by transmitter 100.

Turning now to FIG. 4A, FIG. 4A is a diagram showing elements of outputcontrol circuit 441 suitable for implementation as part of a variablegain control transformer, according to one embodiment of the presentinvention. FIG. 4A shows variable gain control transformer 430 includingprimary winding 432, secondary winding 440, output control circuit 441,and outputs 442 and 444, corresponding to variable gain controltransformer 230 including respective primary winding 232, secondarywinding 240, output control circuit 241, and outputs 242 and 244, inFIG. 2. Also shown in FIG. 4A are variable resistors 446 and 448providing respective variable resistances R1 and R2 and correspondingrespectively to variable resistors 246 and 248, in FIG. 2.

In addition to variable resistors 446 and 448, which are represented asbeing implemented by respective switch pairs 452 a and 452 b, and 452 cand 452 d, the embodiment of output control circuit 441 shown in FIG. 4Aalso includes switches 462, 464, and 466, resistors 467 a and 467 b, andcapacitors 468 a, 468 b, and 469. It is noted that each of the switchingdevices comprised by output control circuit 441 is represented as eitheran NFET or a PFET. For example, switches 464 and 466 are characterizedas PFETs in FIG. 4A, while switches 452 a, 452 b, 452 c, and 452 d(hereinafter “switches 452 a-452 d), and switch 462 are eachcharacterized as an NFET. As was the case for the circuit in FIG. 3,above, however, the representation shown FIG. 4A is merely exemplary,and other embodiments of output control circuit 441 may be arranged andadapted so as to use other types of switching devices.

According to the embodiment shown in FIG. 4A, each of variable resistors446 and 448 are implemented by the source-drain resistances produced bytheir respective paired switches. For example, the resistance presentedby variable resistor 446 corresponds to the source-drain resistancespresented by its constituent plurality of switches 452 a and 452 b, eachof which may be selectively activated independent of the other as shownby control signal CN3 applied to the control terminal of switch 452 aand control signal CN4 applied to the control terminal of switch 452 b.Thus, depending upon whether only one or the other of switches 452 a and452 b is ON, or whether both switches are concurrently ON or OFF, theeffective resistance provided by the source-drain resistances ofswitches 452 a and 452 b allow R1 of variable resistor 446 to beadjusted or varied. Analogously, the operation of switches 452 c and 452d, also respectively controlled by control signals CN3 and CN4, producethe variable resistance R2 provided by variable resistor 448.

Referring to FIG. 4B, FIG. 4B is a diagram showing a sub-circuit forproducing the variable resistances used in the output control circuit ofFIG. 4A, according to one embodiment of the present invention. As shownin FIG. 4B, sub-circuit 450 includes matched current sources 454 a and454 b, high-gain operational amplifier (op-amp) 456, calibrated resistor458, and switch 452. Switch 452 can be seen to correspond to any ofswitches 452 a-452 d in FIG. 4A, for example. Thus, sub-circuit 450 canbe implemented to provide any of the source-drain resistances producedby switches 452 a-452 d.

As further shown in FIG. 4B, the outputs of matched current sources 454a and 454 b are respectively coupled to ground through switch 452 andcalibrated resistor 458 presenting known resistance R_(N), which cancorrespond to any desired resistance R1 or R2. In addition, the outputsof matched current sources 454 a and 454 b respectively feed oppositedifferential inputs to high-gain op-amp 456, while the output ofhigh-gain op-amp 456 provides the control signal CN applied to thecontrol terminal, e.g., gate, of FET switch 452. As a result, switch 452corresponding to any of selectable FETs 452 a-452 d receives a controlsignal CN forcing its source-drain resistance to track the calibratedresistance of calibrated resistor 458 while switch 452 is ON. Moreover,sub-circuit 450 ensures that the source-drain resistance across FETswitch 452 will match the resistance of calibrated resistor 458 for aspectrum current values produced by matching current sources 454 a and454 b. As a result, the source-drain resistance provided by FET switch452 can be seen to be regulated by sub-circuit 450 so as to remainsubstantially constant over process-voltage-temperature (PVT)variations.

The operation of output control circuit 441, and therefore the variablegain control functionality of variable gain control transformer 430, inFIG. 4A, will now be further described by reference to FIG. 5. FIG. 5presents table 500 showing control signals used in conjunction withoutput control circuit 441, in FIG. 4A, for producing variable gaincontrol by transformer 430 in that figure, according to one embodimentof the present invention. Once again we return to the example embodimentin which the PA driver and transmitter comprising variable gaintransformer 430, in FIG. 4A, are configured for multi-mode transmissionusing any one of W-CDMA, GSM, and EDGE transmission modes.

Referring concurrently to FIGS. 4A and 5, according to the embodimentrepresented by table 500, gain control by variable gain controltransformer 430 is only required when transmission occurs in W-CDMAmode, e.g., EDGE and GSM modes require no preamplification signalattenuation by variable gain control transformer 430. As a result, EDGEand GSM transmission signals may be alternately taken from the sameoutput of variable gain control transformer 430, e.g., output 444, whilethe opposite output, e.g., output 442, is AC grounded. That oppositeoutput, e.g., output 442, can be used when W-CDMA transmission signalsare produced, while the output used for EDGE and GSM modes, e.g., output444, is AC grounded during W-CDMA mode operation.

According to table 500, when either of EDGE or GSM modes are in use,control signals CN1, CN2, CN3, and CN4, are LOW, while control signalCN5 is HIGH. Consequently, the constituent NFET switches of variableresistors 446 and 448 are turned OFF, as is NFET switch 462. Switches464 and 466 are shown as PFETs, however, so that a HIGH CN5 applied tothe switch 466 turns that device OFF, while the LOW CN2 signal resultsin PFET switch 464 being ON so as to AC ground output 442. As a resultthe EDGE or GSM mode transmission signals preamplified by the PA drivercan be provided as single-ended outputs to the PA from output 444.

As further indicated by table 500, according to the present embodiment,three variable gain control levels are used in conjunction with W-CDMAmode: (1) maximum gain, e.g., no gain control attenuation, (2)approximately 6 dB of gain control, and (3) approximately 12 dB of gaincontrol. For each variable gain control level provided by output controlcircuit 441, the W-CDMA mode output signals are provided to the PA assingle-ended outputs from output 442.

For maximum gain in W-CDMA mode, control signal CN1 is HIGH, ACgrounding output 444, while control signals CN3 and CN4 are LOW, so thatswitches 452 a-452 d are nominally OFF. In addition, control signal CN2to PFET switch 464 is HIGH, assuring that switch being in an OFF state.Because, despite being nominally OFF, switches 452 a and 452 b ofvariable resistor 446 are AC coupled to output 442 at which relativelylarge voltage swings may occur during W-CDMA mode operation, biasing ofswitches 452 a and 452 b is provided by switch 466 and resistors 467 aand 467 b, which may be approximately twenty kilo-ohm (20 kΩ) resistors,for example. Due to implementation of a PFET as switch 466, a LOWcontrol signal CN5 applied to switch 466 turns that device ON, therebyDC biasing the switches of variable resistor 446 to assure that theirrespective drain/source junction diodes remain reverse-biased duringmaximum gain W-CDMA mode operation.

When either approximately 6 dB or approximately 12 dB of gain control isdesirable during W-CDMA mode operation, each of switches 462, 464, and466 are turned OFF, while the constituent switches of variable resistors446 and 448 are selectably activated to provide the desired amount ofgain control. Thus, when 6 dB of gain control is required, controlsignal CN3 is HIGH, turning NFET switches 452 a and 452 c ON, andcontrol signal CN4 is LOW, turning NFET switches 452 b and 452 d OFF.When, instead, 12 dB of gain control is desired, the status of controlsignals CN3 and CN4 is reversed, turning ON switches 452 b and 452 d,and turning OFF switches 452 a and 452 c. As a result of the describedoperation of output control circuit 441, variable gain controltransformer 430 can provide up to approximately 12 dB ofpreamplification gain control.

Thus, the present application discloses embodiments of a variable gaincontrol transformer suitable for implementation to providepreamplification gain control in a communications transmitter, forexample, a transmitter used as part of a more modern mobilecommunication device transceiver. By introducing an output controlcircuit coupled to the secondary winding of PA driver outputtransformer, for example, embodiments of the present invention enablesuch a transformer to provide preamplification gain control. Inaddition, by utilizing variable resistors to selectably determine thegain control provided by the output control circuit, embodiments of thepresent invention enable variable gain control by the transformer acrossa spectrum of desirable gain control levels. Moreover, by utilizing avariable capacitance tuning circuit at the input of the variable gaincontrol transformer, embodiments of the present invention enable aflexible architecture in which a single transformer may be used toprovide variable gain control while supporting multiple transmissionmodes.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skill in the art would recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. The described embodiments are to beconsidered in all respects as illustrative and not restrictive. Itshould also be understood that the invention is not limited to theparticular embodiments described herein, but is capable of manyrearrangements, modifications, and substitutions without departing fromthe scope of the invention.

The invention claimed is:
 1. A variable gain control transformercomprising: a primary winding connected to differential inputs of saidvariable gain control transformer; a secondary winding configured toprovide a single ended output to a load; an output control circuitcoupled to said secondary winding and comprising first and secondvariable resistors; and a variable capacitance tuning circuit includinga plurality of selectable fixed capacitance unit cells coupled betweensaid differential inputs, wherein said selectable fixed capacitance unitcells comprise first and second fixed unit capacitors each having acorresponding plate selectably coupled to ground by respective first andsecond switches, and said first fixed unit capacitor and said secondfixed unit capacitor are selectively coupled to a supply voltage by acommon third switch.
 2. The variable gain control transformer of claim1, wherein said output control circuit is configured to substantiallymatch an input impedance of said load.
 3. The variable gain controltransformer of claim 1, wherein a first node shared by said first fixedunit capacitor and said first switch, and a second node shared by saidsecond fixed unit capacitor and said second switch are selectivelycoupled to said supply voltage by said common third switch.
 4. Thevariable gain control transformer of claim 1, wherein said outputcontrol circuit is configured to provide up to approximately 12 dB ofgain control.
 5. The variable gain control transformer of claim 1,wherein said first and second variable resistors are implemented byrespective first and second pluralities of source-drain resistancesproduced by respective corresponding first and second pluralities ofselectable field-effect transistors (FETs).
 6. The variable gain controltransformer of claim 5, wherein each of said selectable FETs isconfigured to receive a control signal forcing its respectivesource-drain resistance to track a calibrated resistance while each ofsaid selectable FETs is ON.
 7. The variable gain control transformer ofclaim 1, wherein said variable gain control transformer is implementedas part of a power amplifier (PA) driver circuit in a radio frequency(RF) transmitter.
 8. The variable gain control transformer of claim 7,wherein said variable gain control transformer is configured toselectably support a plurality of transmission modes of said RFtransmitter.
 9. The variable gain control transformer of claim 7,wherein said RF transmitter is implemented as part of an integratedmobile communications transceiver.
 10. A radio frequency (RF)transmitter comprising: a differentially driven power amplifier (PA)driver including a variable gain control transformer for providing asingle ended output to a PA of said radio frequency transmitter; anoutput control circuit comprising first and second variable resistorscoupled to a secondary winding of said variable gain control transformerfor providing a single ended output to drive said PA; and a variablecapacitance tuning circuit including a plurality of selectable fixedcapacitance unit cells coupled between differential inputs of saidvariable gain control transformer, wherein said selectable fixedcapacitance unit cells comprise first and second fixed unit capacitorseach having a corresponding plate selectably coupled to ground byrespective first and second switches, and said first fixed unitcapacitor and said second fixed unit capacitor are selectively coupledto a supply voltage by a common third switch.
 11. The RF transmitter ofclaim 10, wherein said variable gain control transformer is furtherconfigured to substantially match an input impedance of said PA.
 12. TheRF transmitter of claim 10, wherein a first node shared by said firstfixed unit capacitor and said first switch, and a second node shared bysaid second fixed unit capacitor and said second switch are selectivelycoupled to said supply voltage by said common third switch.
 13. The RFtransmitter of claim 10, wherein said output control circuit isconfigured to provide up to approximately 12 dB of gain control.
 14. TheRF transmitter of claim 10, wherein said first and second variableresistors are implemented by respective first and second pluralities ofsource-drain resistances produced by respective corresponding first andsecond pluralities of selectable field-effect transistors (FETs). 15.The RF transmitter of claim 14, wherein each of said selectable FETs isconfigured to receive a control signal forcing its respectivesource-drain resistance to track a calibrated resistance while each ofsaid selectable FETs is ON.
 16. The RF transmitter of claim 10, whereinsaid variable gain control transformer is configured to selectablysupport a plurality of transmission modes of said RF transmitter. 17.The RF transmitter of claim 16, wherein at least one of said pluralityof transmission modes supports both voice-band and data-bandcommunications.
 18. The RF transmitter of claim 10, wherein said RFtransmitter is implemented as part of an integrated mobilecommunications transceiver.
 19. An electronic device comprising: avariable gain control transformer; and a variable capacitance tuningcircuit including a plurality of selectable fixed capacitance unit cellscoupled between differential inputs of said variable gain controltransformer, wherein said selectable fixed capacitance unit cellscomprise first and second fixed unit capacitors each having acorresponding plate selectably coupled to ground by respective first andsecond switches, and said first fixed unit capacitor and said secondfixed unit capacitor are selectively coupled to a supply voltage by athird switch.